During the tenure of this grant I would like to develop an expertise in the use of non-invasive techniques, specifically stable isotopes and NMR spectroscopy to study substrate metabolism and hormone action in-vivo with the ultimate objective of then applying these techniques to the study of normal and diseased states in man. My initial focus will be on substrate cycling in man. Substrate cycling occurs when two opposing non-equilibrium reactions operate simultaneously with the net result of heat production by the hydrolysis of ATP. It has been hypothesized that these cycles may be under hormonal control and serve an important role in the provision of sensitivity and flexibility in metabolic reactions. It has also been hypothesized that futile cycling may play an important role in the regulation of body weight and thermogenesis. The existence of such cycles have been elegantly demonstrated to occur in-vitro. However, in-vivo studies demonstrating their existence are few, and their existence in man has yet to be shown. The specific aims of these studies will be to 1) to determine whether or not substrate cycling of glucose/glucose-6-phosphate and fructose-6-phosphate/fructose 1-6 diphosphate exists in man; 2) To quantitate the contribution of these cycles to basal thermogenisis in man; 3) to determine whether thyroid and other hormones play a role in their regulation. The general principle that will be used in order to determine the various rates of substrate cycling will be one of utilizing a labeled substrate which undergoes a transformation which persists after recycling. I also plan to study the technique of high resolution nuclear magnetic resonance (NMR) as it applies to the study of metabolism in living tissues. The major single advantage NMR offers over existing techniques is its ability to directly observe the intracellular milieu in the living organism. The initial experiments will be correlative between NMR studies with actual in-vitro measurements of isolated liver and heart cell preparations with the ultimate goal of developing NMR techniques to study the metabolism of organs in-vivo. I believe these techniques will prove to be a very useful tool in metabolic studies and that they will have direct clinical applications in human investigation and diagnosis.